Ferrous metal pickling bath and method of preparing same



Patented Aug. 14, 1945 UNITED STATES FERROUS METAL PICKLING BATH AND METHOD OF PREPARING SAME Jan D. Buys, Pittsburg, and Aaron Wachter, San

Francisc o, Calif., assignors to Shell Development Company, San Francisco, Calif., a corporation of Delaware No Drawing. Application August 8, 1942, Serial No. 454,190

6 Claims.

The present invention relates to the art of pickling metals for the removal of rust and scale coating from the surfaces thereof and more particularly to acid pickling baths and methods of preparing the same.

Pickling solutions for removing oxides from the surface of metals ordinarily consist very largely of a dilute solution of an inorganic acid such as sulfuric acid or hydrochloric acid. The acid, although it removes the iron oxides and scale, also attacks the metal to some extent, resulting in excessive loss of the metal to be pickled and high consumption of the pickling acid. It also appears that the hydrogen liberated by the action of the acid on the metal has a serious embrittling eflect upon the metal being treated. For these reasons inhibitors are commonly employed in such solutions to decrease the activity of the acid with respect to the metal without greatly afiecting the solvent action of the solution with respect to the metal oxides.

Many diflerent materials have been employed in the past and are employed at present as additive inhibitors. For example, it has been suggested that crude anthracene and acid extracts of anthracene oil residues with mineral acids be employed for this purpose. It is known that certain compounds of nitrogen, such as the heterocyclic compounds, e. g. compounds contained in commercial pyridine, quinoline and acridine, are eflective as inhibitors in pickling acid baths. Many other organic and inorganic compounds and reaction products thereof have also been used, such as stannous tin, glue, pine oil, alcohols, formaldehyde, arsenic, the reaction product of an aldehyde, a primary amine and carbon bisulfide, the reaction product of a pyridine compound and benzyl chloride, mercaptothiazole, mercaptobenzothiazole and various cation active nitrogencontaining aromatic organic compounds.

All of the above materials are more or less eifective for the purpose of inhibiting the action of pickling acids on the metal which is being treated. However, many oi them also have certain objectionable features such as slowing up the pickling action, producing obnoxious vapors, coating the metal being treated with undesirable deposits or at least requiring special equipment and operative procedure for their preparation and/or isolation which, in turn, makes them relatively expensive for such purposes.

It is an object 01' the present invention to provide an improved pickling bath and method of preparing the same which has all of the requisite beneflcial eflects. A further object of the invention is to prepare such a bath from waste acid sludges of a particular type. A still further object is to provide a method of preparing an improved pickling bath which requires no additional equipment other than that which is ordinarily present where quantities of waste acid of the preferred type are obtained and which is ordinarily utilized for reconcentrating such waste acids. Another object is to provide a pickling acid bath from waste acid sludges which does not require the addition of an inhibitor prior to its use or during its preparation.

A ccording to the present inventionit has been f oundthat alkylation acid sludges, i. e. sulfuric acid which has been used as a catalyst in the alkylation of hydrocarbons, more particularlyin fiilealkylation of hydrocarbons having not more than eight carbon atoms and especially in the alkylation of paraffin and iso-paraiiln hydrocarlions containing not more than eight carbon atoms, contain reaction products which are suitable for use in pickling baths. Alkylation proc's'ses yiaaiag acid sludges of this type are described in U. S. Patents 2,232,674 and 2,211,747. It appears that during alkylation reactions numerous complex side reactions take place yielding. among other materials, hydropolymerization products and various highly unsaturated com pounds as well as various ethers, esters and alcohols, and sulfuric acid derivatives thereof. which of these compounds or others that may be present is responsible for the inhibiting effect is not known, although it has been found the compounds present having suitable inhibition properties are relatively unstable in concentrated sulfuric acid and tend to react therewith or become otherwise decomposed to yield inactive compounds. Thus, alkylation acid sludge which has been withdrawn from alkylation treaters and shipped to steel mills and the like for use as a. pickling acid has been found to be inferior to spent acid newly withdrawn from an alkylation system.

We have now found that spent alkylation acid leaving an alkylator contains quantities of excellent inhibitors, the fugitive tendency of which may be overcome by treating the spent alkylation acid as described hereinafter. Specifically, the

- natural inhibitors present may be stabilized by out substantial harm to the inhibitors present if the acid is kept at a suitably low temperature, for example 0 C. or lower, at which temperatures no decomposition of the inhibitors appears to take place. The dilution is preferably carried out with ice water in order to avoid temperature increases during the dilution step which tend to bring about deactivation of the inhibitOrs.

After the acid has been promptly diluted, it is allowed to stand for a period sufllciently long to permit acid-insoluble organic matter to separate and stratify, after which the separated oily layer may be decanted or otherwise removed. In the event excessively large quantities of acid-insoluble organic matter are present in the particular acid being utilized, the acid may be promptly diluted and thereafter further treated in the manner described in U. 8. Patent 1,793,146 in relation to pressure distillate sludges, i. e. by heating after dilution to further reduce the organic content.

The inhibited action of alkylation acid which has been treated as described above is clearly demonstrated by the data of the following examples:

Exmrr-s 1 Steel strips were immersed in technical grade sulfuric acid and in alkylation acid which had been treated as described above. The conditions and results of the test are given in Table 1.

It will be noted that the corrosion rate in the instance of the technical acid was of the order of twenty times that of the inhibited alkylaticn acid of the same concentration.

Exnlrts II Although acid prepared according to the present invention may be utilized at any desired concentration and temperature, pickling baths for use in steel rolling mills and the like are usually maintained at concentrations between approximately 5% and H2804 and at temperatures between about 160 F. and 180 F. In Table 2 the -results are given of tests in which commercial operation conditions were duplicated. Steel strips were immersed in C. P. sulfuric acid, aikylation acid and inhibited C. P. sulfuric acid, all of the acids having a concentration of 8% by weight, the temperature being held at approximately 172 F. The inhibited C. P. sulfuric acid was prepared by adding the recommended quantity .5 gms. per liter) of a commercial inhibitor of the cation active nitrogen-containing type, sold under the trade-name of Rodine 1, to an 8% solution of C. P. sulfin'ic acid. After 11 minutes immersion, the steel strips were withdrawn. washed and dried. The effects of the various acid baths were determined by weight differentials, the surface area of each of the test strips being 4.19 square inches.

Table 2 Weight loss Weight loss Test in grams in gmsJsq. in.

8% C. P. H1504 0. 1691 0.0403 8% C. P. H2804 inhlbi 0.1258 .0302 8% alkylation H4804 0. 1181 .0282

It will be noted that under commercial operating conditions the results obtained using alkylation acid were of the same order and in fact somewhat superior to those obtained when using a standard commercial inhibitor.

Although prompt dilution to 50% H2804 content is preferred, dilution to other concentrations within the approximate range of 40% to 60% H2804 will be found satisfactory in many instances. The optimum dilution will of course vary somewhat with different alkylation acids and should preferably be determined experimentally in each case. However, dilution in one step to concentrations normally used in pickling baths, i. e. 5% to 10% H2804, has been found to be undesirable in view of the fact that a large portion of the organic material present in alkylation acid as it leaves the alkylation system is water-soluble. It has therefore been found preferable to dilute to approximately 50% H2804, since in most instances it will be seen that the solubility of the oil is at a minimum at concentrations of this order and hence better separation of organic matter is thus attained.

The prompt dilution step described above has also been found to greatly enhance the separation of organic material from spent alkylation acid. For example, a particular acid was allowed to stand for approximately six weeks without being diluted. It was then diluted to approximately 50% H2804 concentration, the organic material separating at this concentration removed and the acid diluted to 7% H2804 for use as a pickling bath. This material was found to be unsuitable for pickling metal which was to be tinned due to separation of asphaltic materials which adhered to the pickled surfaces of the metal. Another sample of similar acid which has been promptly diluted to 50% H2804 content, the organic material being separated and the acid then diluted to 7% H2804 content, did not throw out more than a trace of organic matter after the second dilution step.

As an example of the complete pr n of a spent alkylation acid containing a relatively large amount of organic material difficult to separate, the following may be cited: A particular spent acid was found to contain approximately 89% H1804, 6% to 7% organic matter and the remainder water. This acid was diluted to 50% E804 concentration with water, the organic matter separating carefully removed and the remaining acid treated for a period of approximately 2 hours at a temperature of 350 F. and a pressure of 60 pounds. The pressure cooking resulted in the separation of approximately 2% more organic matter, which also was carefully removed. The acid was then diluted to 8% H2804 concentration and found to have pickling properties equivalent to acid baths inhib'ted with standard pickling inhibitors and in many other respects superior.

In those instances where the metal to be treated in a pickling bath is not to be later plated or porcelain enameled, or in cases where it is practical to solvent-wash the metal surfaces after the pickling treatment, it is not necessary to remove the last traces of insoluble organic matter from the pickling solution as its deleterious effect is confined primarily to staining the metal surfaces.

The inhibited acid produced according to the present invention may either be used alone or blended with other inhibited or pure acids, the beneficial inhibiting effect remaining to a large extent when allqlation acid prepared according to the manner described above is blended with as much as by volume of H2804 derived from usual commercial sources.

We claim as our invention:

1. In a method of preparing a ferrous me 1 pickling bath the steps comprising promptl diluting sulfuric acid which has been used as an alkylation catalyst in the alkylation of saturated hydrocarbons containing not more than eight carbon atoms to a concentration of the order of 50% by weight H2504 whereby a portion of the organic content thereof is rendered insoluble therein, permitting said dilute acid to stand for a period of time sufficient to stratify said insoluble organic matter, removing said insoluble organic material, heating said sulfuric acid whereby a further portion of the organic content of said sulfuric acid is rendered insoluble therein, removing said last mentioned insoluble organic material and thereafter further diluting said sulfuric acid to a concentration of from approximately to approximately by weight.

2. In a method of preparing a ferrous metal pickling bath, the steps comprising promptly diluting sulfuric acid which has been used as an alkylation catalyst in the alkylation of saturated hydrocarbons containing not more than eight carbon atoms to a concentration of the order of 50% by weight H2804 whereby a portion of the organic content thereof is rendered insoluble therein, permitting said dilute acid to stand for a period of time suflicient to stratify said insoluble organic matter, removing said insoluble organic material and thereafter further diluting said sulfuric acid to a concentration of from approximately 5% to approximately 10% by weight.

3. In a method of preparing a ferrous metal pickling bath, the steps comprising promptly diluting sulfuric acid which has been used as a catalyst in the alkylation of saturated hydrocarbons containing not more than eight carbon atoms to a concentration of from approximately 40% to approximately 60% by weight H2804 whereby a portion of the organic content of said sulfuric acid is rendered insoluble therein, permitting said dilute acid to stand for a period of time sufiicient to stratify said insoluble organic matter, removing said insoluble organic material and thereafter further diluting said sulfuric acid to a concentration of from approximately 5% to approximately 10% by weight.

4. In a method for preparing a ferrous metal pickling bath, the steps comprising promptly diluting sulfuric acid which has been used as a catalyst in the alkylation of saturated hydrocarbons containing not more than eight carbon atoms to a concentration of from approximately 40% to approximately 60% by weight H2804 whereby a portion of the organic content of said sulfuric acid is rendered insoluble therein, permitting said dilute acid to stand for a period of time sufiicient to stratify said insoluble organic matter, said acid being maintained prior to dilution at a temperature below that at which substantial reaction between said acid and the organic content thereof occurs, said temperature being not higher than approximately 0 C., removing said insoluble or- 1 ganic material and thereafter further diluting said sulfuric acid to a. concentration of approximately 5% to approximately 10% by weight.

5. A ferrous metal pickling bath comprising essentially sulfuric acid, water and a material recovered from promptly diluted sulfuric acid which has been used as a catalyst in the alkylation of saturated hydrocarbons containing not more than eight carbon atoms, said material comprising sulfuric acid and such organic substances as are normally present in spent sulfuric acid alkylation catalyst and are soluble in sulfuric acid having a concentration of not more than approximately 60% by weight H2804 and being substantially free from such organic substances as are normally present in spent sulfuric acid alkylation catalyst and are insoluble in sulfuric acid having a concentration between approximately 40% and 60% by weight H2504, said pickling bath containing not more than approximately 60% by weight H3804.

6. The process of pickling metals which comprises subjecting the metal to the action of a bath comprising sulfuric acid, water and a material recovered from promptly diluted sulfuric acid which has been used as a catalyst in the alkylation of saturated hydrocarbons containing not more than eight carbon atoms, said material comprising sulfuric acid and such organic substances as are normally present in the spent sulfuric acid alkylation catalyst and are soluble in sulfuric acid having a concentration of not more than approximately by weight H2804 and being substantially free from such organic substances as are normally present in spent sulfuric acid alkylation catalyst and are insoluble in sulfuric acid having a concentration between approximately 40% and 60% by weight H2804, said pickling bath containing not more than approxim mately 60% by weight H2804.

JAN D. RU'YS. AARON WACH'IER. 

